1. Field of the Invention
The present invention relates to a method of forming a diamond film used for electronic devices and sensors, and in particular, to a method of forming a diamond film without delamination from a substrate, in which a large area diamond film can be obtained at a low cost.
2. Description of the Background
Since diamond has excellent heat-resistance and is the hardest of the known substances, it is used for abrasion resistant sections of cutting tools, and the like. Also, diamond has a wide band gap of approximately 5.5 eV, and generally has good insulating properties, although it can function as a semiconductor when doped with impurities. Also, diamond has excellent electrical characteristics, such as a high dielectric breakdown voltage, a high saturation velocity, and a low dielectric constant. Therefore, diamond is expected to be a suitable material for electronic devices and sensors which are used at high temperatures, at high frequencies, and in high electric fields. So far, diamond has been applied to the following fields: a material for an optical sensor or a light emitting device for the short wavelength range of ultraviolet rays or the like, because of the wide band gap; a material for a heat dissipating substrate because of its high thermal conductivity and low specific heat; a surface acoustic wave device because of its high propagation velocity of sound waves; and an X-ray window or optical material because of its high transmittance and refractive index.
In order to maximize the characteristics or performance of diamond in various applications as described above, it is required that a single-crystal diamond film of high quality, in which the defects of the crystal structure are reduced or a coalesced diamond film having coalesced grain boundaries, can be synthesized. Also, a large area diamond film of high quality must be obtained at a low cost in order to implement the applications of the diamond film. Conventionally, single crystals of diamond have been obtained by mining or by synthesis at high temperatures and pressures. However, the resultant diamond film or grain has an area of approximately 1 cm2 at the largest, and the price is very high. Therefore, the fields in which diamond films are industrially used are limited to polishing powder and as a tool edge for high precision cutting tools.
Vapor phase syntheses of diamond films include, for example, the micro-wave chemical vapor deposition (CVD) method disclosed in Japanese Patent Publication Nos. 59-27754 and 61-3320, the high-frequency plasma CVD method, the hot filament CVD method, the direct-current plasma CVD method, the plasma-jet method, combustion method, and the thermal CVD method. In accordance with these vapor phase processes, a large area diamond film can be obtained at a low cost.
In order to industrially implement electronic devices and sensors using diamond films, diamond films having excellent electrical properties must be obtained. That is, a method must be established for synthesizing a single crystal or a coalesced film having a significantly low grain boundary density.
So far, however, a diamond film that is formed by the vapor phase processes on non-diamond substrates such as silicon, is polycrystalline, in which diamond grains are randomly aggregated with a high grain boundary density. If a diamond film has grain boundaries, carriers (i.e., electrons and holes) may be trapped or scattered, resulting in a deterioration of electrical properties, and thus electronic devices and sensors formed from them do not have practical performance characteristics. Also, when there are grain boundaries in a diamond film, light scattering occurs at the grain boundaries, and therefore the transmittance decreases. Also, chipping occurs easily if a diamond film having grain boundaries is used as an abrasion-resistant material.
By using single crystal synthetic diamond or cubic boron nitride as a substrate, a single-crystal diamond film can be formed by a vapor phase process. However, since there is no synthetic diamond or cubic boron nitride having a large area, a large area single-crystal diamond film cannot be formed by the vapor phase process using these substrates.
Recently, a method for synthesizing a single-crystal diamond film on a single crystal platinum substrate was disclosed by Y. Shintani in J. Materials Research, 11, 2955 (1996). However, the single, crystal platinum substrate used in the method is very expensive, and the diamond film obtained is only less than approximately 1 inch in diameter.
A metal film such as platinum is known to grow on in the same crystalline orientation as the oxide substrate such as strontium titanate (epitaxial growth). Therefore, in order to obtain a large area of single-crystal diamond film at a low cost, a method of synthesizing a diamond film on a substrate which includes platinum grown on an oxide such as strontium titanate was developed as disclosed in Japanese Patent Laid-Open No. 9-48693.
However, although a single-crystal diamond film synthesis is achieved by the method described above, the following problems may occur: generally, a diamond film is synthesized at a high temperature of approximately 800xc2x0 C. When a substrate having a different thermal expansion coefficient from that of a diamond film is used, and then cooled down to room temperature, significantly high stress occurs at the interface between the substrate and the diamond film. The stress causes delamination, cracks, or the like, in the diamond film. Since the thermal expansion coefficient of an oxide such as strontium titanate is significantly different from that of diamond, a diamond film grown on such a substrate easily delaminates from the substrate.
Also, if copper is used as a substrate, since the expansion coefficient of copper is more than ten times as large as that of diamond, delamination of the diamond film occurs, when the diamond film is formed by the vapor phase process at a high temperature of 600xc2x0 C. or above and the resultant diamond film is cooled down to room temperature, as disclosed by J. F. Denatale, et al., in J. Materials Science, Vol. 27, p.553 (1992).
The present invention overcomes the difficulties noted above. An object of the invention is to provide a method for forming a diamond film in which a large area diamond film of a single crystal or having coalesced grain boundaries can be formed by a vapor phase process at a low cost.
A method of forming a diamond film in accordance with the present invention includes the steps of providing trenches for preventing delamination of the diamond film from the surface of a base, forming a metal film on the base to fabricate a substrate, and synthesizing a diamond film on the substrate.
Another method of forming a diamond film in accordance with the present invention includes the steps of forming a metal film on a base, providing trenches for preventing delamination of the diamond film from the surface of the metal film to fabricate a substrate, and synthesizing a diamond film on the substrate.
Preferably, each of the trenches has a width of 0.25 to 50 xcexcm, and there is a space of 1 to 1,000 xcexcm between the adjacent trenches for preventing delamination. Also, preferably, the base is composed of at least one material selected from the group consisting of lithium fluoride, calcium fluoride, magnesium oxide, nickel oxide, aluminum oxide, strontium titanate, barium titanate, lead titanate, potassium tantalate, lithium niobate, yttrium oxide, quartz, and silicon. Also included are solid solutions thereof. Additionally, the base may include a plurality of layers, and an outermost layer of the base is preferably composed of at least one material selected from the group mentioned above.
Also, the metal film is preferably composed of at least one metal selected from the group consisting of platinum, iridium, cobalt, nickel, and iron. Also included are alloys thereof. Also, the substrate may have trenches for separating devices at a larger width than that of the trenches for preventing delamination.
In accordance with the present invention, trenches for preventing delamination are provided on the surface of a base, and a diamond film is formed on a substrate obtained by forming a metal film on the base, or a metal film is formed on a base, a diamond film is formed on a substrate provided with trenches for preventing delamination from the surface of the metal film. Since the trenches reduce the regions in which the diamond film and the substrate are in contact with each other, the stress at the interface between the substrate and the diamond film can be dispersed. As mentioned above, in accordance with the present invention, since the substrate is partitioned by trenches, the generated stress does not exceed the adhesion between the diamond film and the substrate. Thus, delamination, cracks, or the like, do not occur in the diamond film even if the diamond film synthesized at high temperatures is cooled on the substrate, and thus a large area single-crystal diamond film or a coalesced diamond film can be formed at a low cost.